<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<style>

    body{
        margin: 0px;
    }
    img {
        border: 0;
    }
    ul{
        padding: 0;
        margin: 0;
        list-style: none;
    }
    .clearfix:after {
        content: ".";
        display: block;
        height: 0;
        clear: both;
        visibility: hidden;
    }

    .wrap{
        width: 980px;
        margin: 0 auto;
    }

    .pg-header{
        background-color: #303a40;
        -webkit-box-shadow: 0 2px 5px rgba(0,0,0,.2);
        -moz-box-shadow: 0 2px 5px rgba(0,0,0,.2);
        box-shadow: 0 2px 5px rgba(0,0,0,.2);
    }
    .pg-header .logo{
        float: left;
        padding:5px 10px 5px 0px;
    }
    .pg-header .logo img{
        vertical-align: middle;
        width: 110px;
        height: 40px;

    }
    .pg-header .nav{
        line-height: 50px;
    }
    .pg-header .nav ul li{
        float: left;
    }
    .pg-header .nav ul li a{
        display: block;
        color: #ccc;
        padding: 0 20px;
        text-decoration: none;
        font-size: 14px;
    }
    .pg-header .nav ul li a:hover{
        color: #fff;
        background-color: #425a66;
    }
    .pg-body{

    }
    .pg-body .catalog{
        position: absolute;
        top:60px;
        width: 200px;
        background-color: #fafafa;
        bottom: 0px;
    }
    .pg-body .catalog.fixed{
        position: fixed;
        top:10px;
    }

    .pg-body .catalog .catalog-item.active{
        color: #fff;
        background-color: #425a66;
    }

    .pg-body .content{
        position: absolute;
        top:60px;
        width: 700px;
        margin-left: 210px;
        background-color: #fafafa;
        overflow: auto;
    }
    .pg-body .content .section{
        height: 500px;
    }
</style>
<body onscroll="ScrollEvent();">
<div class="pg-header">
    <div class="wrap clearfix">
        <div class="logo">
            <a href="#">
                <img src="http://core.pc.lietou-static.com/revs/images/common/logo_7012c4a4.pn">
            </a>
        </div>
        <div class="nav">
            <ul>
                <li>
                    <a  href="#">首页</a>
                </li>
                <li>
                    <a  href="#">功能一</a>
                </li>
                <li>
                    <a  href="#">功能二</a>
                </li>
            </ul>
        </div>

    </div>
</div>
<div class="pg-body">
    <div class="wrap">
        <div class="catalog">
            <div class="catalog-item" auto-to="function1"><a>第1张</a></div>
            <div class="catalog-item" auto-to="function2"><a>第2张</a></div>
            <div class="catalog-item" auto-to="function3"><a>第3张</a></div>
        </div>
        <div class="content">
            <div menu="function1" class="section">
                <h1>第一章</h1>
            </div>
            <div menu="function2" class="section">
                <h1>第二章</h1>
            </div>
            <div menu="function3" class="section">
                <h1>第三章</h1>
            </div>
        </div>
    </div>

</div>
    <script>
        function ScrollEvent(){
            var bodyScrollTop = document.body.scrollTop;
            if(bodyScrollTop>50){
                document.getElementsByClassName('catalog')[0].classList.add('fixed');
            }else{
                document.getElementsByClassName('catalog')[0].classList.remove('fixed');
            }

        }
    </script>
</body>
</html>

Demo-滚动固定